1. Field of the Invention
The invention relates to a process for producing a strong bond between copper layers, which have been applied by means of thermal spraying of pulverulent copper or copper alloys, and ceramic.
2. Description of Related Art
The bonding of copper as highly electrically conductive metal to oxide ceramic as readily thermally conductive insulator is of particular industrial importance in the field of electronic components. The copper layer is used as strip-conductor layer, electrode layer, sensor layer or else as connecting layer in the soldering of oxide ceramic as heat sink onto other components which develop a particularly high amount of heat.
Various processes are already known for the direct application of copper to ceramic. DE-A-38 24 249 describes, for example, deposition without external current after prior toughening of the ceramic surface and the subsequent palladium nucleation with subsequent heat treatment. This process has the disadvantage of allowing only metal layers having a maximum thickness of up to 5 .mu.m.
DE-A-30 36 128 describes the direct copper bonding process (DCB), in which a copper sheet having a thickness of from 100 to 900 .mu.m is applied to the ceramic surface at a temperature of about 1070.degree. C. The DCB process is, however, only economic if layers having thicknesses of more than 100 .mu.m are to be applied.
A base metallization by means of cathode atomization in vacuo or as described in DE-A-28 24 249 and subsequent electrolytic deposition of copper can be economically carried out for a layer thickness of up to a maximum of 50 .mu.m. The bond strength between the applied layer and the ceramic material can be, in the case of cathode atomization, increased by a so-called coupling layer of chromium, titanium, inter alia, but this coupling layer leads to considerable disadvantages in the further processing of the strip conductors by etching.
Application of copper by sputtering in a vacuum chamber with a prior plasma etching process in the same vacuum chamber is economically possible only up to a layer thickness of from 1 to 2 .mu.m.
In thermal spraying, it is known that copper can be applied both to metallic and to nonmetallic materials. Uses of such coatings are the improvement of the contact resistance or the application of solderable compounds and the like to the materials. The layers thus applied have, owing to the oxidation of the copper during the application process, a high electrical resistance in comparison to pure electrolytic copper. The various thermal spraying processes in vacuo or in chambers of protective gas are technically very complicated and can hardly be economically used in mass production, having regard to the component geometry of the electronic components.
In addition, the exceptionally high dimensional accuracy of the surface geometry required in modern high technology is only possible by means of further costly mechanical treatment in the known process steps, for some parts it cannot be achieved at all.